The invention relates to a control valve, in particular an angle control valve and double control valve, also as a seat valve and angle seat valve, for extreme control applications with erosive and corrosive media at temperatures of over 500° C. and pressure drops of over 100 bar, preferably for draining or down-regulating for high-pressure hot separators, medium-pressure hot separators and vacuum distillation columns, in particular for hydrogenation, and particularly for coal liquefaction, essentially with an inlet funnel, an outlet funnel lined with tungsten-carbide, with valve seats arranged between them, and a valve piston provided with a control valve head, whereby the valve seat and the valve heads consist of solid tungsten carbide.
In typical hydrogenation or ‘upgrading’ plants, heavy hydrocarbons are enriched with hydrogen, in order to produce light hydrocarbons, which are suitable for transport machinery, in particular for aeroplanes and motor vehicles. The reaction between hydrogen and hydrocarbon takes place under high pressure and at high temperatures, and results in a mixture, whose components can then be separated into different levels, namely in high-pressure hot separators, medium-pressure hot separators, as well as in atmospheric and vacuum distillation columns.
For this purpose, control valves are used in critical media with extreme conditions, e.g. in order to drain in high-pressure hot separators, i.e. to down-regulate. In this process, the pressure between the levels is removed, and the flow of the medium is controlled.
Evaporation inevitably takes place in the control valve, partly supercritical, since the pressure falls under the vapour pressure in each valve. This causes cavitation, erosion and vibration in the valves, and possibly in the pipelines, which is exacerbated by the presence of solids, such as, for example, catalyst residues. The most critical application is in coal liquefaction, in which coal provides the heavy hydrocarbons. The ash does not react with the hydrogen and dramatically increases the solids content. It is also very erosive.
The medium flows with high pressure and is reduced from approximately 200 bar to approximately 40 bar. In this process, the medium evaporates and destroys parts of the valve seat, the valve head and the valve stem or valve piston. Since the medium generally contains solids, the solids are flung against the internal walls of the valve, and cause abrasion. In addition, the vibrations in the pipeline are very high, and therefore there is a high degree of destruction of the valve parts. This also happens since the usually conical valve head of the valve piston hits against the valve seat.
The valve housing parts of the control valve normally consist of steel or stainless steel. In the majority of cases, the temperature of the medium is very high, for example, 500° C. Additionally, the control valves comprise a valve seat and a valve head of solid tungsten carbide, and so do not only have a coating. The valve piston consists of stainless steel. However, a valve head made of a tungsten carbide cone cannot easily be welded to a valve piston made of steel. Thus, a two-part ring made of stainless steel connects both parts and is then welded, which is expensive. A disc spring, which is heat-resistant, i.e. high-temperature resistant, made of Inconel presses the valve piston against the valve head, when the steel parts expand due to the high temperature.
Steel expands considerably at this high temperature—however, tungsten carbide does not, since this material has a much smaller thermal expansion coefficient. When the control valve is opened, strong flow forces occur, in such a way that the valve piston is exposed to vibrations. At the same time, parts of the valve piston and the valve head shake in the valve seat. In order to reduce these vibration effects, damping rings made of graphite were used in the outlet funnel, which however did not have the desired effect, and dissolved after a while.
Amongst others, further problems occur due to the fact that the tungsten carbide interior parts used sit in the housing parts made of steel. Relatively large gaps form due to the different thermal expansion coefficients, which makes the tungsten carbide parts subject to vibrations. The medium then collects between the gaps which form. The parts stick when cooling. Therefore, it is very difficult to disassemble the control valve to replace the parts which are stuck together.
Furthermore, other tungsten carbide parts, such as the valve head, are tightly bonded with steel parts, such as the valve piston, which is very complicated for the reasons mentioned. Tungsten carbide is also very expensive and has long delivery times.
So-called distribution valves are also known, in which the medium is distributed in a housing. However, double control valves, i.e. two control valves in one housing are not known.